Glycans are integral components of biological systems with far reaching activities, many of which are only beginning to be understood. Glycans constitute the most abundant and diverse class of biomolecules found in natural systems, consisting of oligosaccharide chains that are present as independent polysaccharides (e.g., cellulose, an important structural component in plants; and heparin sulfate, an import factor of blood clotting in mammals) or as glycoconjugates with lipids (glycolipids), proteins (glycoproteins, proteoglycans), and small molecule natural products (e.g., antibiotics such as erythromycin, vancomycin, and teicoplanin).
Glycans play a role in almost every aspect of cellular activity. Most glycans in higher eukaryotes are produced in the secretory pathway by glycosylation events, which entail the enzymatic transfer of saccharides or oligosaccharide chains onto lipids and proteins. Protein glycosylation is a complex co- or post-translational process that modifies the majority of the human proteome and serves a vast array of biological functions. Protein glycosylation exerts intrinsic effects on structure, from mediating folding and oligomerization, to increasing stability, solubility, and circulation time. Inside of the cell, glycans affect recognition, binding, targeting, and cellular distribution. At the cell surface, glycans are prominently displayed where they are involved in a host of molecular recognition events that modulate important physiological processes, such as cell-cell adhesion, inflammation, angiogenesis, coagulation, embryogenesis, differentiation, communication, and a myriad of other cellular signaling pathways.
Cell surface glycans have also been associated with physiological dysfunctions such as bacterial and viral infection, rheumatoid arthritis, and tumor progression. In the latter case, several types of oncofetal and aberrant glycans have been established to correlate with malignancy, invasiveness, inflammation and cancer metastasis. In particular, altered terminal fucosylation and sialylation, which are believed to result from changes in expression locations and levels of fucosyltransferases (an enzyme that transfers a fucose from a donor substrate to an acceptor substrate, a glycoconjugate or glycan) and sialyltransferases (an enzyme that transfers a sialic acid from a donor substrate to an acceptor substrate, a glycoconjugate or glycan) respectively, are associated with tumor malignancy. For example, glycan determinants like Lewis y, Lewis x, sialyl Lewis x, sialyl Lewis a, sialyl Tn, Globo H, fucosyl GM1, and polysialic acid are expressed at elevated levels in neoplastic tissues. For this reason, these epitopes are promising and eagerly pursued targets for glycan-based vaccines. Additionally, several congenital glycosylation disorders, lysosomal storage disorders, and immunological diseases have been linked with dysregulation of glycan catabolism/metabolism. Although known to be involved in physiological and pathophysiological events, the identification of many glycan structures and delineation of their mode of action at the molecular level has been complicated by their underpinning complexity.
Glycan complexity results from many factors. They are synthesized in a non-templated, post-translational process, which means that sites of glycoconjugate glycosylation and structures within them have proven, thus far, to be minimally predicatable. This also means that glycans cannot be genetically manipulated in a similar fashion to DNA and proteins. Glycans are synthesized in the secretory pathway by a suite of enzymes that are subject to multifaceted controls. The end glycan products can have enormous structural complexity (many possible glycan structures, the diversity of which is also a function of the sugar building blocks), structural micro-heterogeneity (multiple different glycan structures attached to a glycoconjugate at the same position), and structural macro-heterogeneity (multiple sites and types of glycan attachment; for example, glycoproteins can be N-linked at Asn residues, or O-linked at Ser/Thr resides). Heterogeneity in glycan structures appears to be dynamically regulated and functionally significant, governing multivalent interactions the cell surface. Heterogeneity and multivalency complicate structure-function studies and the isolation of homogenous glycans in meaningful amounts from natural sources is nearly impossible. For the procurement of homogenous glycoconjugates/glycans synthesis is the only viable route, but remains one of the most formidable challenges in glycobiology.
The link between glycan activity and complexity has presented major challenges to deciphering their activities on an individual protein, let alone, proteomic scale. Among the challenges facing global analysis are development of general methods for isolating glycans from complex proteomes; determining saccharide composition, site of protein modification, and fraction occupancy; and understanding the direct roles of glycans in cellular function and dysfunction.
Specific glycan-tagging systems provide a powerful method for probing the structure of heterogeneous glycans. The key to glycan tagging entails incorporating modified sugars derivatized with chemical reporting groups into cellular glycans (typically via the normal biosynthetic pathways, a process known as metabolic oligosaccharide engineering, or MOE) and then detecting the tagged-glicans by labeling their chemical reporting groups with a complementary probe that chemically reacts with them in a specific manner. Many selective chemical probing techniques have been used for performing chemistry with chemical reporting group-tagged glycoconjugates in cells. These methods include bioorthogonal reactions such as ketoneaminooxy/hydrazide ligation, Staudinger ligation, Michael addition, and the strain-promoted, and Cu(I)-catalyzed [3+2] azide-alkyne cycloaddition (CuAAC). Several chemical reporting groups are tolerated and successfully incorporated into glycoconjugates using MOE, including ketones, thiols, photoreactive groups, azides, and alkynes. These reporting sugars have been labeled with tags such as FLAG peptides, biotin, and fluorescent or fluorogenic molecules. The strength of these systems is that the labeled glycan products have the potential to be manipulated for specific glycan studies involving: enrichment and glycoproteomic analysis by means of mass spectrometry detection and/or quantitation by flow cytometry or visualization through microscopy to obtain information about glycan localization, trafficking, and dynamics.
The incorporation of exogenous natural or unnatural sugars into glycans has been achieved by cellular biosynthetic pathways. These processes involve multistep enzymatic transformations that render free sugars in the cytosol into nucleotide-donor sugars, the substrates for glycosyltransferases. In the case of fucose (Fuc), a salvage pathway consisting of Fuc kinase and GDP-Fuc (guanosine diphosphate fucose) pyrophosphorylase contributes to the production of GDP-Fuc, which is then exploited by fucosyltransferases (FucTs) located in the Golgi apparatus to add Fuc onto glycoconjugates. Modifications at the 6-position of Fuc are tolerated by the salvage pathway and FucTs. In the sialic acid (NeuAc) biosynthetic pathway, the precursor N-acetylmannosamine (ManNAc) is derived from GlcNAc or UDP-GlcNAc through specific epimerases, then sequentially converted to sialic acid by the cytosolic enzymes ManNAc 6-kinase, sialic acid-9-phosphate synthase, and sialic acid-9-phosphate phosphatase. CMP-NeuAc is subsequently formed in the nucleus, and transported to the Golgi apparatus for glycan elaboration by sialyltransferases. Studies on metabolic delivery of mannosamine or ManNAc analogs show that N-acyl chains up to five carbon atoms long are tolerated by the sialic acid biosynthetic pathway.
The incorporation of exogenous natural or unnatural sugars comprising less toxic probes into glycans by cellular biosynthetic pathways is would be important to the study of glycosylation is likely to provide useful information for diagnosis and disease prognosis, in addition to unveiling new therapeutic targets.